THE CHAMPIONS LEAGUE
The Champions League is in some means a "super league" since the best teams in every league feature in it. But then with a new Super League, the Champions league would be diminished.

Part of the appeal, especially in recent times, has been the successes of Real Madrid, Barcelona, Bayern, etc. With these clubs in the new alleged league, then we'd see no triumphs of Ronaldo, Messi, et al.
Yes, there are many other teams in the competition. But as a competition between the best, it would be gutted if the very best won't participate in it.
UEFA naturally won't like this and would fight tooth and nail to protect their prime asset. The Euros, whilst lucrative, are only held once every four years. The Champions League as an annual competition brings in far more sponsorship revenues, and this surely would be diverted to the Super League over the Champions League.